Method for separating off nitrogen and hydrogen from natural gas

ABSTRACT

The invention relates to a method for separating off nitrogen and lighter components, in particular hydrogen, carbon monoxide, neon and argon, from a feed fraction (e.g., natural gas) that is to be liquefied containing at least methane, nitrogen and hydrogen. The cooling and liquefaction of the feed fraction proceeds against the refrigerant or mixed refrigerant of at least one refrigeration cycle. In the inventive method, the feed fraction ( 1 ) is partially condensed (E 1 ), and separated in at least one rectification column (T) into a methane-rich fraction ( 6 ) and a fraction ( 4 ) containing nitrogen and lighter components. The methane-rich fraction ( 6 ) is subcooled. Additionally, cooling of the top condenser (E 2 ) of the rectification column (T) proceeds via a refrigerant or mixed refrigerant or a substream of the refrigerant or mixed refrigerant of at least one, refrigeration cycle ( 20 - 24 ).

SUMMARY OF THE INVENTION

The invention relates to a method for separating off nitrogen andlighter components, in particular hydrogen, carbon monoxide, neon andargon, from a feed fraction that is to be liquefied containing at leastmethane, nitrogen and hydrogen, wherein the cooling and liquefaction ofthe feed fraction proceeds against the refrigerant or mixed refrigerantof at least one refrigeration cycle.

To obtain liquefied methane (LNG), in the recent past, new “natural gassources” have been utilized. These new natural gas sources are, forexample, coal gasification or methanization. The gas mixtures obtainedfrom these processes contain, in addition to the main component methane,hydrogen in amounts up to above 20% by volume, and also nitrogen inamounts of greater than 10% by volume. Furthermore, these gas mixturescan contain traces of carbon monoxide, carbon dioxide, neon and argon.In addition to the nitrogen, the hydrogen must also be separated fromthe methane that is to be liquefied in order to be able to maintain therequired specifications for the LNG product. It is a problem thatalthough the hydrogen boils comparatively low, it is very highly solublein the liquefied methane.

If the LNG product, as is customarily the case, is temporarily stored,the nitrogen content must not exceed a value of 1% by volume, sinceotherwise an unwanted roll-over effect can occur in the LNG storagetank.

It is an object of the present invention to provide a method of the typein question for separating off nitrogen and lighter components from afeed fraction that is to be liquefied containing at least methane,nitrogen and hydrogen, which method firstly makes possible adequateseparation of the nitrogen and secondly makes possible simultaneousseparation of hydrogen, preferably at a comparatively high pressure.

Upon further study of the specification and appended claims, otherobjects and advantages of the invention will become apparent.

To achieve these objects, a method is proposed for separating offnitrogen and lighter components from a feed fraction that is to beliquefied containing at least methane, nitrogen and hydrogen, whichmethod is characterized in that

-   -   a) the feed fraction is partially condensed,    -   b) is separated in at least one rectification column into a        methane-rich fraction and a fraction containing nitrogen and        lighter components, and    -   c) the methane-rich fraction is subcooled,    -   d) wherein the cooling of the top condenser of the rectification        column proceeds via a refrigerant or mixed refrigerant or a        substream of the refrigerant or mixed refrigerant of the, or at        least one, refrigeration cycle.

According to the invention, nitrogen present in the feed fraction, aswell as lighter components likewise present, are separated from the feedfraction together in a rectification column. Since the feed fraction,before liquefaction thereof, is usually compressed to a pressure between40 and 90 bar and, before being fed into the rectification column, isthrottled to a pressure between 20 and 40 bar, the components that areseparated off can be obtained at a comparatively high pressure. Inparticular in the case of the relatively valuable component hydrogen,obtaining it at high pressure is desirable, since the hydrogen isgenerally fed to further use. Furthermore, it is advantageous in theprocedure according to the invention that, owing to the integration ofthe separation of lighter components into the nitrogen separationprocess, the capital costs which would otherwise be necessary for aseparate separation of the lighter components can be saved.

Further advantageous embodiments of the method according to theinvention for separating off nitrogen and lighter components from a feedfraction that is to be liquefied containing at least methane, nitrogenand hydrogen are characterized in that

-   -   the feed fraction has a methane content between 50 and 90% by        volume, a nitrogen content between 1 and 30% by volume and a        hydrogen content between 1 and 30% by volume,    -   the fraction containing nitrogen and lighter components is        warmed against the feed fraction that is to be cooled, wherein        this cooling proceeds before and/or after an optionally provided        drying of the feed fraction,    -   the condenser temperature of the top condenser of the        rectification column can be varied,    -   if the liquefied methane-rich fraction is stored temporarily,        tank boil-off gas occurring during the temporary storage is        compressed and fed to the feed fraction that is to be liquefied,    -   a substream of the feed fraction that is to be liquefied is fed        to the rectification column as stripping gas stream in the        bottom phase of the rectification column and/or is cooled in the        reboiler of the rectification column and fed to the        rectification column as a further feed stream, and    -   if the cooling and liquefaction of the feed fraction proceeds        against refrigerants and/or mixed refrigerants of at least two        refrigerant cycles, the top condenser of the rectification        column is cooled by means of a separate refrigeration cycle or        by at least a substream of the refrigerant or mixed refrigerant        of the lightest refrigeration cycle, wherein this advantageous        embodiment of the method according to the invention is suitable,        in particular, for LNG plants of relatively large capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated schematically with reference to anexemplary embodiment in the drawing and will be described extensivelyhereinafter with reference to the drawing. Various other features andattendant advantages of the present invention will be more fullyappreciated as the same becomes better understood when considered inconjunction with the accompanying drawing wherein:

the FIGURE illustrates an embodiment according to the invention.

As illustrated in the FIGURE, the feed fraction containing at leastmethane, nitrogen and hydrogen is fed via line 1 to an optionallyprovided prepurification unit R. This comprises, at low pressures of thefeed fraction, a compression to a pressure between 40 and 90 bar, andalso generally a removal of carbon dioxide and mercury and also drying.The feed fraction pretreated in this manner is then fed via line 2 toheat exchanger E1 and cooled and partially condensed therein. The heatexchanger E1 is usually constructed as a plate heat exchanger or as ahelically coiled heat exchanger. In the case of correspondingly largecapacities, if appropriate, a plurality of heat exchangers, arranged inparallel to one another and/or in series, are provided.

The feed fraction is cooled and liquefied against at least onerefrigeration cycle of any desired design which is only shownschematically in the figure by the pipe sections 20 to 24 which will beconsidered in more detail hereinafter. This refrigeration cycle ispreferably constructed as an expander cycle or mixed cycle.

The partially condensed feed fraction is fed, via line 3 in which anexpansion valve a can be provided, to rectification column T and thereinseparated into a liquid fraction and a gas fraction. At the top of therectification column T, a fraction containing nitrogen and also thelighter components, in particular hydrogen, is removed via line 4. Thisfraction is warmed in the heat exchanger E1 against the feed fractionthat is to be cooled and subsequently released at the battery limits vialine 5. Alternatively, or in supplementation thereto, the fractionremoved via line 4 can also be used for precooling the feed fraction 1before drying thereof in R, in order to reduce the water content of thefeed fraction 1 before the drying in R.

When the feed fraction 1 that is to be liquefied contains heavyhydrocarbons, preferably upstream of the rectification column T, theseheavy hydrocarbons may be separated off. This separation can proceed ina heavy hydrocarbon separator, a deethanizer, a depropanizer, etc.

From the bottom phase of the rectification column T, via line 6, amethane-rich liquid fraction having a nitrogen content of typically lessthan 3% by volume is removed and subcooled in the heat exchanger E1against the refrigerant or mixed refrigerant 23 of the refrigerationcycle. Via line 7, the LNG product obtained in this manner is fed, afterexpansion in valve b, to a storage tank S.

Tank boil-off gas produced within the storage tank S is removed from thestorage tank S via line 8, compressed, if necessary, in a single ormultistage manner in compressor V, and fed to the feed fraction 1,preferably before the cooling thereof. Alternatively, the tank boil-offgas can also be fed to a combustion gas system or released at thebattery limits.

The top condenser E2 of the rectification column T is cooled accordingto the invention by a refrigerant or mixed refrigerant or a substream ofthe refrigerant or of the mixed refrigerant. In the embodiment shown inthe FIGURE, some of the (mixed) refrigerant stream is fed via line 22 tothe top condenser E2 and after passage through same is conducted vialine 23 through the heat exchanger E1. The (mixed) refrigerant warmed inheat exchanger E1, against the feed fraction that is to be cooled, andis then removed from the heat exchanger E1 via line 24. The substream ofthe refrigerant or mixed refrigerant that is not fed to the topcondenser E2 is conducted via line 21, in which a control valve d isprovided, and combined with substream 22 in line 23.

The top condenser E2 can be constructed either as a plate exchanger,helically coiled heat exchanger or TEMA (Tubular Exchanger ManufacturersAssociation) exchanger and can be arranged in or above the rectificationcolumn T, wherein an arrangement above the rectification column T makesa reflux pump obsolete. By varying the column height and the condensertemperature, the methane content in the fraction removed via line 4 canbe adjusted virtually as desired, namely between approximately 10 ppmand some % by volume.

Advantageously, via line 9, a substream of the feed fraction that is tobe liquefied is conducted through the reboiler E3 of the rectificationcolumn T, via which the nitrogen content of the LNG product stored inthe storage tank S can be kept below 1% by volume, then expanded in thevalve c and fed via line 9′ to the rectification column T as anadditional feed stream. Alternatively thereto, a substream of the feedfraction that is to be liquefied can also be used as stripping gasstream in the bottom phase of the rectification column T before or aftercooling has been performed in the heat exchanger E1, or a substream ofthe (mixed) refrigerant can be used as a heat source for the reboilerE3.

The method according to the invention for separating off nitrogen andlighter components from a feed fraction that is to be liquefiedcontaining at least methane, nitrogen and hydrogen can be usedadvantageously at nitrogen and hydrogen contents up to in each case 30%by volume. In this case, comparatively high hydrogen contents in thefeed fraction that is to be liquefied have only a subsidiary effect onthe total energy requirement of the method according to the invention,since some of the cold is recovered by warming the fraction taken offfrom the top of the rectification column T that contains nitrogen andalso the lighter components, in particular hydrogen.

The entire disclosure[s] of all applications, patents and publications,cited herein and of corresponding German Application No. DE102010044646.7, filed Sep. 7, 2010 are incorporated by reference herein.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

1. A method for separating off nitrogen and lighter components from afeed fraction that is to be liquefied containing at least methane,nitrogen and hydrogen, comprising: a) partially condensing the feedfraction (1) (E1), b) separating the partially condensed feed fractionin at least one rectification column (T), having a top condenser (E2),into a methane-rich fraction (6) and a fraction (4) containing nitrogenand lighter components, and c) subcooling the methane-rich fraction (6),wherein cooling of said top condenser (E2) of said rectification column(T) is performed using a refrigerant or mixed refrigerant or a substreamof the refrigerant or mixed refrigerant of at least one refrigerationcycle (20-24).
 2. The method according to claim 1, wherein said feedfraction (1) is natural gas.
 3. The method according to claim 1, whereinthe partial condensing of said feed fraction (1) is performed in a heatexchanger (E1) by heat exchange with said refrigerant or mixedrefrigerant of at least one refrigeration cycle.
 4. The method accordingto claim 1, wherein said feed fraction (1) has a methane content of50-90% by volume, a nitrogen content of 1-30% by volume, and a hydrogencontent of 1-30% by volume.
 5. The method according to claim 1, whereinsaid fraction (4) containing nitrogen and lighter components is warmed(E1) against the feed fraction (1, 2) that is to be cooled.
 6. Themethod according to claim 5, further comprising drying the feedfraction, wherein cooling of the feed fraction (1) proceeds before saiddrying (R) of the feed fraction (1).
 7. The method according to claim 5,further comprising drying the feed fraction, wherein cooling of the feedfraction (1) proceeds after said drying (R) of the feed fraction (1). 8.The method according to claim 1, wherein the condenser temperature ofsaid top condenser (E2) of said rectification column (T) can be varied.9. The method according to claim 1, wherein subcooling of themethane-rich fraction yields a liquefied methane-rich fraction, which istemporarily stored, and tank boil-off gas (8) occurring during thetemporary storage (S) of said liquefied methane-rich fraction iscompressed (V) and fed to the feed fraction (1) that is to be liquefied.10. The method according to claim 1, wherein a substream of the feedfraction (1) that is to be liquefied (a) is fed to the bottom portion ofsaid rectification column (T) as a stripping gas stream, and/or (b) iscooled in a reboiler (E3) of said rectification column (T) and fed tothe rectification column (T) as a further feed stream.
 11. The methodaccording to claim 1, wherein the cooling and liquefaction of said feedfraction proceeds against refrigerant and/or mixed refrigerants of atleast two refrigeration cycles, and said top condenser (E2) of saidrectification column (T) is cooled by means of a separate refrigerationcycle or by at least a substream of the refrigerant or mixed refrigerantof the lightest refrigeration cycle.